1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance imaging apparatus having a device for measuring magnetic field strengths.
2. Description of the Prior Art
Among other things, magnetic field strengths of nuclear magnetic resonance signals for generating magnetic resonance images are measured in a magnetic resonance apparatus. To that end, the apparatus has, for example, a reception antenna fashioned as an electrical coil. In order to avoid local current densities in the reception antenna that would place a patient at risk during an RF transmission phase as a result of resonances, particularly given a local reception coil, the reception antenna is detuned or turned off in a complicated way during an RF transmission phase. Further, the coil must necessarily be fashioned of electrically conductive materials and thus disturbs the homogeneity of the basic magnetic field that is important for the image quality. Further, a gradient system of the apparatus generates rapidly switched magnetic gradient fields in the nuclear magnetic resonance apparatus for generating images.
The abstract of Japanese Application 13 16 676 discloses a device for optical magnetic field measurement based on the Faraday effect, wherein an immediate digitalization of measured results takes place without interposition of electrical analog-to-digital converters. To that end, a light source emits light that is supplied via a mirror arrangement and a polarization device to three optical paths that are formed of magneto-optical material, exhibiting a length ratio of 1:2:4 relative to one another, and which are exposed to a magnetic field to be measured. The light emerging from the paths passes through another polarization device having electrical outputs connected to comparators. An angular difference of, for example, 45.degree. thereby exists between the polarization planes of the polarization devices. The electrical output levels of the comparators form a discrete quantity bit-by-bit whose value corresponds to the field strength of the magnetic field to be measured.
German PS 33 26 736 discloses a magnetic field measuring device likewise based on the Faraday effect and containing a Faraday cell. In one embodiment, the Faraday cell is arranged in a solenoid-like coil arrangement, and in a further embodiment, the Faraday cell is fashioned as an optical monomode fiber.
European Application 0 086 373 discloses a magnetic field measuring device based on the Faraday effect that is optimized in view of space requirements. Lead glass, bismuth silicon oxide, bismuth germanium oxide or yttrium iron silicate (Y.sub.3 Fe.sub.5 O.sub.12) are thereby proposed as magneto-optical materials.